Recently, there has been an increase in a need for a high-speed data transmission, and an OFDM cellular system is adopted as a transmission system for a variety of high speed transmission system due to the compatibility with such high-speed transmission. The OFDM system may be equipped with a smart antenna for an increase in a received signal to noise ratio, an expansion of service coverage, and an increase in a base station processing capacity, and interference cancellation between cells in a cellular system having a frequency reuse factor 1.
However, in cases where a conventional OFDM cellular system makes use of such a smart antenna, there exists a need for supporting mixed users with smart antenna and omnidirectional antennas and also problems such as a coverage mismatch between data channels and broadcast information. Generally, such problems can be solved by employing a method that divides a smart antenna area and an omnidirectional antenna area by symbol units, and a method that transmits broadcast information using beams of smart antenna over multiple time slots to improve cell coverage.
To achieve an initial connection and operation of a mobile station, however, basic operation parameters of a system need to be transmitted as broadcast signals from a base station. In a communication system adopting a smart antenna, typically, a base station uses a plurality of antennas and a mobile station utilizes a single antenna. Therefore, signals of the mobile station first need to be detected on an uplink in order to calculate an antenna weight vector for an adaptive beamforming in the base station.
However, for the initial connection and operation of the mobile station, the mobile station must acquire basic operation parameters such as broadcast signals for a downlink. This makes it difficult for the base station to transmit such basic system operation parameters through an adaptive beamforming.
To overcome such drawbacks, generally, the base station tends to repeatedly transmit initial basic system operation parameters by time or frequency units through switched beams in preset multiple directions.
However, the above-described method has a disadvantage in that loss of data can be caused by the collision between subcarriers of adjacent cells in cases where beams emitted from each base station in a multiple cell environment are superimposed each other.
In an OFDM cellular system using a smart antenna, broadcast information is transmitted over multiple time slots using switched beams to overcome a coverage mismatch between data channels and broadcast information. In such a case, however, beams are generally switched in a predetermined order, which might cause interferences due to a superposition of such beams.
As a prior art for prevention of superimposition of beams of neighboring cells, U.S. Pat. No. 6,795,413 registered Sep. 21, 2004, entitled “Radio communications system in which traffic is transmitted on the broadcast channel” discloses a method for transmitting data via a broadcast channel in a radio communications system adopting a space division multiple access (SDMA). This prior art seeks to prevent interferences between cells by prohibiting transmission of adjacent cells in a process of transmitting broadcast information wherein one base station transmits broadcast information only during one frame in a superframe. The above-captioned prior art has an advantage in that interferences between adjacent cells during transmission of broadcast information are prevented, but a disadvantage in that much time consumption needs until registration of mobile station. On the contrary, the present invention proposes a method that takes into account simultaneous transmission of broadcast information from all of cells and then applies a beam selection technique to prevent interferences between cells that may be occurred therein. Thus, the present invention can complete the initial registration during a short time under a condition where interferences between adjacent cells hardly occur.